DEMM Engineering and Manufacturing

The culmination of years of work by the Timaru District Council to improve the way the district’s sewage was piped and treated was reached today with the awarding of a $15 million contract to upgrade the domestic waste water treatment plant on Aorangi Road at Seadown. The contract, awarded by the council to Downer NZ Ltd, will see major earthworks over a total of approximately 48 hectares on both sides of Aorangi Road and a number of separate projects that will achieve the on-site separation and treatment of industrial and domestic wastewater flows. More than 30 hectares of primary oxidation ponds, eight hectares of maturation ponds and 10 hectares of wetlands will be constructed over the next 18 months.
A key sub-contractor to Downer, Rooney Earthmoving, will carry out the construction of the oxidation pond system, which accounts for 30 per cent of the project. Announcing the contract awarding today, council drainage and water manager Grant Hall said that although the earthworks were extensive, they were minimised by the construction method to be employed. “The ponds are to have an impermeable liner of silt first harvested from the site of the excavation and stored along with top soil until later being returned to the ponds once their shape has been formed,” Mr Hall said. The silt liner will be to a depth of half a metre and protected with a layer of coarse aggregate, he said. Planting, landscaping and beautification of the area would follow.
The project will mean a significant change to the way the district’s sewage was treated. Industrial flows will continue to be treated in the existing milliscreen plant, but domestic flows will be re-routed from the existing plant and treated in a new wastewater plant to be built alongside. “The treated flows would then recombine, be filtered in the wetlands and then discharged through the existing ocean outfall,” Mr Hall said. Sewage from the towns of Pleasant Point, Geraldine and Temuka will be piped directly into the new maturation pond.
The work follows a five-stage upgrade of the main trunk sewer over the last few years to provide separate waste streams to the Aorangi Road plant. Mr Hall said the latest project was a significant component of the overall Timaru District Wastewater Management Strategy. “Along with the need for us to meet resource consent conditions for the discharge of our sewage to the sea over the next 35 years, the community has been calling for environmental improvements as part of the wastewater strategy,” Mr Hall said.
“One of the key parts is being able to separate industrial waste from domestic waste and treat them separately. The treatment costs of the combined flow otherwise would have been astronomical.” The separation also achieved greatly increased sewage capacity for the district and would provide for growth in both population and industrial development over the next 30 years, he said.
The council used a robust selection process to establish the winning contractor, Mr Hall said. Tenders, of which there were four with the highest being $23 million, were evaluated using a weighted formula measuring contractor experience, track record, methodology and price. The council was assisted in the selection process by three senior consulting engineers from Beca. Budgets for the domestic plant upgrade have been included in the current Long Term Plan and the draft proposed Long Term Plan.